In blasting operations, it is becoming increasingly important to efficiently utilize the blasting power of the explosives. It is important to achieve the maximum breakage of rock into the correct sizes for a given amount of explosive material. It is further becoming increasingly important to minimize the effects of blasting on nearby structures by reducing the amplitude of ground vibration produced by a blast. The principle method for achieving these objectives is to time the ignition of a plurality of explosive units which are placed at selected locations within an area of operation. This separates the total weight of explosives over the area and makes it possible to separate the time of ignition between the explosive units. Such separation can improve breakage and reduce the peak ground vibrations. The delay time is measured on the order of milliseconds.
Both electric and nonelectric delay blasting caps and circuits have been developed. The nonelectric units are typically based upon the burning time for a pyrotechnic mixture. However, such burning time has the limitation in its accuracy. This limitation of accuracy prevents the optimum timing of the ignition events.
Numerous electronic detonation circuits have been proposed to achieve greater accuracy over pyrotechnic devices. Representative ones of such circuits are shown in the following patents. U.S. Pat. No. 4,825,765 to Ochi et al., U.S. Pat. No. 4,730,558 to Florin et al., U.S. Pat. No. 4,712,477 to Aikou et al., U.S. Pat. No. 4,646,640 to Florin et al., U.S. Pat. No. 4,633,779 to Bitts et al., U.S. Pat. No. 4,632,031 to Jarrott et al., U.S. Pat. No. 4,586,437 to Miki et al., U.S. Pat. No. 4,445,435 to Oswald, U.S. Pat. No. 4,395,950 to Oswald, U.S. Pat. No. 4,324,182 to Kirby et al., U.S. Pat. No. 4,328,751 to Oswald, U.S. Pat. No. 4,311,096 to Oswald, U.S. Pat. No. 4,145,970 to Hedberd et al., U.S. Pat. No. 4,136,617 to Fowler, U.S. Pat. No. 4,095,527 to Moore, III, U.S. Pat. No. 3,851,589 to Meyer.
A principal problem encountered in the use of electronic delay detonators is the accuracy of the clock signal within the delay circuit itself. There are numerous constraints upon the use of a local oscillator within the delay circuits. First, the oscillator must be relatively inexpensive to justify the one time use in an explosives application. Further, such an oscillator should not be subjected to rate variations as a result of changing pressures, which will be experienced when one unit receives the pressure wave from the ignition of another unit. Further, fluctuations in ambient temperature, humidity and pressure can cause an oscillator to vary in its rate.
Various techniques have been proposed for calibrating local oscillators in electronic delay detonator circuits. These are set forth in the following patents. U.S. Pat. No. 4,674,047 to Tyler et al., U.S. Pat. No. 4,419,933 to Kirby et al., and U.S. Pat. No. 3,500,746 to Abrosini et al.
It can therefore be seen that it is important to have a method for calibrating the local oscillator within a delay circuit so that accurate delay times can be achieved while at the same time doing so in such a manner that is reliable and rapid.